(1) Field of the Invention
This invention relates to an apparatus and method for mixing dry powders in a liquid and more particularly, to an apparatus and method by which a water soluble powder or polymer, may be directly and uniformly dissolved in water in high concentrations without excessive agitation.
(2) Description of the Prior Art
The reduced drag observed in liquid flows when small quantities of polymer are present in the fluid has created a need for mixing the polymer powder uniformly in the carrier fluid. Water soluble resin (WSR) polymers such as Polyox.TM. brand of polyethylene-oxide, (trademark of Union Carbide), have been used for many years in such a drag reducing role and have developed a good performance record. In the past, however, WSR polymer mixing was a two step process. First, the powder is dispersed in glycerol (in which Polyox will not dissolve), then the Polyoxglycerol slurry is introduced into the carrier fluid (usually water).
At the present time, however, glycerol which is utilized as the emulsion fluid, is considered a hazardous material, requiring an environmental impact statement for any technique which ejects or utilizes glycerol as a drag reducing agent. This is operationally as well as cost prohibitive, and has lead to a requirement for mixing the polymer directly into the carrier fluid which in many cases is fresh water.
In the direct mixing prior art, the mixing of the polymer has been accomplished by mixing the polymer powder with water by hand, carefully and slowly stirring the solution until the necessary concentration was reached. This is extremely difficult because for a uniformly distributed solution of concentrated polymer and water, the local molecular concentration during mixing cannot exceed 1% percent at anytime. When the local concentration exceeds 1%, the polymer coagulates into a very viscus gum which will not dissolve easily into the solution without employing high shear mixing techniques.
The drag reducing effectiveness of the polymer depends on the high molecular weight, or size, of the long chain polymer molecules. If the long chains are broken, as can occur through high shear mixing, the drag reducing performance of the solution is drastically diminished. Hence, there is a need for a low shear, WSR polymer water mixing device and method which can disburse the polymer more sufficiently to keep local molecular concentration below 1%.
Prior art devices and methods for directly mixing powders or suitable water soluble resin polymers into water include U.S. Pat. No. 4,077,612 which pre-wet the resin powder in a turbulent pre-wetting chamber, and then discharge the wetted resin into a mixing vat above the surface of the water, such that the wetted resin falls down into the water.
Another prior art device and method includes the type disclosed in U.S. Pat. No. 3,752,446 which completely dissolves the resin in turbulent mixing chambers by providing a stream of air carrying resin particles which is turbulently carried in a spray of water.
A further prior art device such as disclosed in U.S. Pat. No. 4,685,810, by Sakuichi, et al. uses a high pressure air feed to feed the powder resin into a water vat below the surface of the water using high pressure jets. The high pressure jets inject the powder resin into the water and agitate the water to mix the resin with the water. A single, high pressure jet is not sufficient, however, to assure local concentrations do not exceed 1%. In addition, at high polymer concentrations, Non-Newton effects reduce air bubble breakdown, allowing dry powder to escape through the upper surface. This requires additional low level agitation and an evenly distributed array of turbulent jets in the tank.
Due to the violently turbulent nature of the mixing of the above prior art devices, high sheer stresses are created that breakdown the long chains of polymers in the resin resulting in a solution with drastically diminished drag reducing capabilities.